Fluoropolymers are used as materials for various products in a wide range of industries including automotive industry, semiconductor industry, chemical industry, and paint industry due to their advantages such as prominent chemical resistance, solvent resistance, heat resistance, and stain resistance.
Fluoropolymers are produced by emulsion polymerization, suspension polymerization, or solution polymerization of fluoroolefins. In the case of emulsion polymerization, surfactants are typically used. As more surfactant is used, more polymer particles are produced in early stages of the emulsion polymerization, resulting in a higher polymerization rate and improved productivity of a fluoropolymer. However, too much surfactant often results in a fluoropolymer that is poor in properties such as water resistance. This problem has created a need for the development of production methods by which polymerization is efficiently carried out in the presence of a small amount of a surfactant without causing adverse effects on the properties of a resulting fluoropolymer.
In response to the above-mentioned situation, a method for producing a fluoropolymer using a linear aliphatic sulfonate surfactant has been proposed (Patent Document 1). This method aims to replace expensive ammonium perfluorooctanoate, which is commonly used in emulsion polymerization of fluoropolymers, with a linear aliphatic sulfonate surfactant. However, one disadvantage of this method is that it produces only a small number of particles.
Also, the following methods were proposed: production methods using an alkylphosphoric acid or an ester thereof as a non-fluorous surfactant (Patent Documents 2 and 3); and a method using a compound containing a quaternary carbon atom to which groups, such as phosphoric acid, sulfonic acid, and carboxylic acid groups, are attached (Patent Document 4).
However, the use of an alkylphosphoric acid or an ester thereof does not completely ensure sufficient levels in terms of the number of produced particles, the polymerization rate, the molecular weight of resulting polymers, the polymer concentration of dispersions, the polymerization temperature, the polymerization pressure, and the like. Besides, a further increase in particle production is required of the method using a compound containing a quaternary carbon atom to which groups, such as phosphoric acid, sulfonic acid, and carboxylic acid groups, are attached.    Patent Document 1: U.S. Pat. No. 6,512,063    Patent Document 2: US 2007/0032591    Patent Document 3: US 2007/0018783    Patent Document 4: WO 2005/063827